How to copy data from one armature to another POSE MODE
This should work: go in pose mode (arm. A), edit the pose then select all bones, and use ctrl-c to "copy pose", then select the other armature (arm. B), go in pose mode, select all bones and press ctrl-v (paste pose). Edit mode af arm B remains the same as before.

I tried on simple armatures, but I suspect it needs two armatures with the same bones and structure...

if you need to make arm A like its pose, start from pose mode, and then select all bones and press ctrl-A > apply pose as rest pose: it will stay the same in edit mode...